Steel Cruciform Sample with Nitrided Arms Achieves Higher Plastic Strain in the Gauge Region

This paper describes a novel solution to increase plastic strain in the gauge region of a cruciform sample during a forming limit test. A nitriding procedure was used to increase the strength of the arms of the specimen and at the same time enabled higher plastic deformation in the centre of the sample. DC 5 steel sheets were cut in bone-shape samples and subjected to the nitriding procedure. Uniaxial tensile tests were done to obtain the properties of both the raw and thermo-chemically treated material. Two shapes of the gauge region, partially protected against diffusion of nitrogen, were modelled with the use of Abaqus software and a numerical analysis of biaxial tensile tests were conducted. Based on the obtained numerical analysis results DC 5 steel cruciform samples were nitrided while keeping the same gauge region geometries and subsequently subjected to a biaxial tensile test. The obtained results prove the positive influence of the nitriding procedure on increasing the strength of the cruciform sample arms and at the same time the plastic strain in the centre of the sample. The test bench analysis showed almost six times higher plastic deformation, as compared to the raw specimen, however special attention should be paid to the nitriding process parameters and the shape of the protected gauge region.